A. Field of the Invention
This invention relates to non-toxic solid gas generants, particularly those suitable for the production of substantially pure nitrogen gas, and more particularly to the use of high nitrogen content compounds, preferably containing no hydrogen, such as the dialkali salts of bitetrazole or azobitetrazole or the like as a base for such generants
B. Description of the Prior Art
There has been considerable interest in the generation of nitrogen gas for a number of purposes including the inflation of aircraft or automobile safety crash bags, also termed air bags, and the purging of fuel lines and storage tanks for reactive or pyrogallic fuels as used in liquid fueled rocket motors.
The details of crash bag systems have been widely discussed, as have the reasons for the selection of pyrotechnic devices for the rapid and dependable generation of gases for inflating the bag. The operational constraints of crash bags are also well known. The system must supply non-toxic gas to inflate the bag because air bag systems generally vent into the passenger compartment on deflation, and because of the very real probability of bag rupture in an actual crash situation. Additionally, the gas must inflate the bag at a temperature which the vehicle occupant can tolerate. The time period for attainment of maximum inflation has been determined to be from 20 to 100 milliseconds. The device must be safe to handle and store prior to production. It must be adaptable to mass production line installation techniques and not introduce an unreasonable hazard then or during the life of the vehicle. It must assure reliable operation during the life of the vehicle containing it, which may be on the order of 10 years or longer.
The objectives of rapid generation of cool non-toxic inflation gas and long-term operability depend to a large extent on the gas generant selected and the physical form into which it is initially compounded.
If a suitable propellant can be designed, then the design of a complete passive restraint system undertaken with consideration of the characteristics of a particular propellant stands a better chance of practical success.
Naturally, from every point of view, the most desirable atmosphere inside an inflated crash bag would correspond in composition to the air outside it. This has thus far proven impractical of attainment. The next best solution is inflation with a physiologically inert or at least innocuous gas. The most practical of these gases has proven to be nitrogen.
The most successful of the prior art solid gas generants of nitrogen that are capable of sustained combustion have been based upon the decomposition of compounds of alkali metal, alkaline earth metal and aluminum derivatives of hydrazoic acid, especially sodium azide. Such a nitrogen gas generant comprising mixtures of alkali metal azides, metal and metalloid oxides, molybdenum disulfide, and optionally sulfur, pressed into pellets is disclosed in U.S. Pat. No. 4,203,787 that was granted on May 20, 1980 to George F. Kirchoff and Fred E. Schneiter.
There are disadvantages, however, to the use of these azides, particularly in the generation of the inflating gas for air bag systems.
Sodium azide, a Class B explosive, is a highly toxic material. It is easily hydrolyzed, forming hydrazoic acid which is not only a highly toxic and explosive gas, but it also readily reacts with heavy metals such as copper, lead, etc. to form extremely sensitive solids that are subject to unexpected ignition or detonation. Especial handling in the manufacture, storage and eventual disposal is required to safely handle such materials and gas generants prepared from the azide compounds.
A number of other nitrogen gas generants have been reported, as disclosed, for example, in U.S. Pat. Nos. 3,004,959, 3,055,911, 3,171,249, 3,719,604, 3,814,694, 3,873,477 and 3,912,561. Many of the prior art gas generants are based on nitrogen-containing compounds such as those derived from the various hydroxamine acid and hydroxylamine derivatives, while others consist of various polymeric binders, hydrocarbons and carbohydrates which are oxidized to produce non-corrosive and, often termed, "non-toxic" gases. The gas products from these compositions contain unacceptably high levels of CO.sub.2, CO and water for use in air bag applications where the possibility exists that the occupant may breathe, even for short periods of time, high concentrations of the gases produced from the gas generator. These compositions do not meet the requirements that the combustion products meet industrial standards for toxic and other gases such as CO, CO.sub.2, etc.